The invention is in the field of Semiconductor devices, and relates more specifically to a method of making devices such as MOSFETS and diodes, including SOI devices, suitable for high-voltage and power applications.
In fabricating high-voltage power devices, tradeoffs and compromises must typically be made in areas such as breakdown voltage, size, conduction losses and manufacturing simplicity and reliability. Frequently, improving one parameter, such as breakdown voltage, will result in the degradation of another parameter, such as conduction losses. Ideally, such devices would feature superior characteristics in all areas, with a minimum of operational and fabrication drawbacks.
Improvements over a basic structure, in which increased breakdown voltages are achieved by providing a linear doping profile in the drift region, are shown in related U.S. Pat. No. 5,246,870 and U.S. Pat. No. 5,412,241, both commonly assigned with the instant application and incorporated herein by reference. In these SOI devices, the drift region between the channel and drain in a lateral MOS structure is provided with various features, such as a thinned portion and a linear lateral doping density profile, which result in substantially increased breakdown voltage characteristics. Additionally, a top field plate is provided over a field oxide of essentially constant thickness to permit twice the conducting charge to be placed in the drift region, thereby reducing conduction losses without reducing breakdown voltage. However, to maintain high breakdown voltage, the total amount of conduction charge near the source side of the drift region must be kept very small, thereby leading to a bottleneck for current flow and preventing optimum reduction in conduction losses.
Another improvement over the basic SOI structure is shown in U.S. Pat. No. 5,648,671, also commonly assigned with the instant application and incorporated herein by reference. This patent shows a lateral thin-film SOI device with a linearly-graded field oxide region and a linear doping profile, features which serve to reduce conduction losses without reducing breakdown voltage. Yet another improved high-voltage thin-film device is disclosed in U.S. patent application Ser. No. 08/998,048, commonly assigned with the instant application, co-invented by the present inventors and incorporated herein by reference. This application discloses another technique for improving such devices, employing a step oxide region of intermediate thickness to increase current-carrying capability while maintaining a high breakdown voltage.
While all of the foregoing structures offer an improvement over standard SOI structures, they still suffer from the drawback that they cannot operate effectively and efficiently at high current levels in the source-follower mode, wherein a "source-high" bias condition may be encountered during operation and a high breakdown voltage must be maintained in a device which must also handle high current levels.
A lateral thin-film SOI device configuration of the type described above, but in which operation, and particularly operation of a MOSFET device in the source-follower mode, is enhanced by significantly increasing permissible saturated current flow and reducing the on resistance of the device structure while maintaining high breakdown voltage capability, is described in allowed U.S. patent application Ser. No. 09/100,832, filed Jun. 19, 1998 by the present inventors, commonly assigned with the instant application, and incorporated herein by reference in its entirety. The advantages of the foregoing structures are achieved by providing the disclosed structures with both a graded top oxide and a graded drift region. While devices of this type can be made using conventional prior-art doping and LOCOS techniques, as noted in Applicants' earlier application, such prior-art techniques are relatively complex, expensive and time consuming.
Accordingly, it would be desirable to have a method of making semiconductor devices with a graded top oxide and a graded drift region using a relatively simple, economical and rapid fabrication process.